The present invention relates to the application of a deposit of material to a succession of discrete articles. More particularly, the present invention relates to a method and apparatus for depositing a droplet or an extended bead of thermoplastic or thermoset material onto the surface of a metal or plastic part. Once applied in accordance with the present invention, the flexible compressible bead extends above the surface of the part in order to create assembly resistance when the part is assembled into or over another part. This resistance serves to temporarily secure the location of the respective parts one to another prior to more permanent joining by welding, gluing or threaded torquing.
Many parts that are used in ultimate assembly in industries such as the automotive industry require some partial manual assembly prior to the ultimate incorporation of these parts into finished goods. For example, rear wheel drive axles of certain automobile manufacturers require the use of bolts coupled with lock washers as part of their assembly. Presently, employees must manually assemble the lock washers to the bolts in preparation for ultimate installation of the combined parts on an assembly line. As can be appreciated, the labor costs associated with manually assembling these lock washers to the bolts are rather significant. Furthermore, once the lock washers are assembled to the bolts there is no structure provided to keep them retained on the bolts pending final assembly. As a result, the washers often fall off in the box on the way to the final assembly line. If this occurs then the axles are assembled with washers missing and serious problems in the ultimate assembly can be created.
In another example, automobile companies have begun utilizing many brackets made from metal stampings which are attached to vehicles by several screws. The brackets and screws are currently shipped separately to the assembly plant under separate part numbers. Once they arrive at the installation facility, they must be coupled prior to installation. As a result, significant additional time and labor costs are incurred to combine the brackets and screws once they arrive at the installation plant. There is always the danger that one or more of the screws may fall out of the assembly prior to ultimate installation or through human error fail to initially be inserted in the appropriate place.
Prior attempts to meet these needs have proven inadequate. That is because the requirements for retaining the individual parts together as a single unit prior to assembly are multifaceted. In particular, it is required that any system that is used to accomplish this temporary retaining purpose cannot interfere with or alter the final assembly of the parts. For example, the structure used to accomplish the retaining function cannot alter the seating torques required to achieve a desired clamp load. Thus, there is a complicated balancing act between providing a retaining material which is tough enough to resist part disassembly, yet not change or interfere with the final assembly.
The prior art does not provide a completely adequate solution to this relatively recent assembly problem. For example, U.S. Pat. No. 4,851,175 to Wallace discloses a method of making O-rings by supplying a continuous stream of liquid hot melt material under the force of gravity alone onto a rotating spindle or directly upon the shank of the rotating fastener. This method, however, is capable of forming only a continuous O-ring around the outer circumference of the fastener and generally uses a heater such as a flame jet spaced from a falling filament of material to soften the deposit on each fastener to cause it to flow into a more conforming state as required such as a flatter wider band.
In addition, this prior art method contemplates a continuous filament of hot liquid material falling from a nozzle that is not capable of precisely locating a dot of such material on only a portion of the outer circumference of such fasteners. As a result, this method is only effective in producing O-rings that cover the entire 360.degree. circumference of a portion of a fastener. Such an O-ring is usually intended to effect the final assembly of parts by acting as a seal or the like. Such an O-ring would be insufficient in many instances to provide a deposit of material that is tough enough to resist part disassembly, but does not interfere with or alter the final assembly of the parts. This prior art method likewise does not provide for discontinuous flow of material that only activates in the presence of a fastener.
It is likewise known to apply a patch of resilient thermoplastic material on a portion or all of the circumference of a selected portion of a fastener such as described in U.S. Pat. No. 3,787,222 to Duffy et al. The material deposited, however, acts not to temporarily retain two parts such as a bolt and washer in place, but rather to increase the resistance between two mating threaded parts in a final assembly and make them self locking so that they will have substantially increased resistance to uncoupling due to vibration and the like.
Another known method of applying thermoplastic material to substrates is disclosed, for example, in U.S. Pat. No. Reissue 33,766 to Duffy et al. This method applies a masking insulating or lubricating coating of teflon or similar material to all or a portion of the threads of the coating. However, the coating produced by this method does not extend far enough above the surface of the fastener or have sufficient retaining ability in order to serve as a retaining element for a second part. Both of these above described methods produce materials that tend to closely follow the contours of the threads of the fastener when applied and also interfere with the ultimate assembly of the parts.
It is apparent, therefore, that there is need to be able to form discrete deposits of material onto the surface of a part over 360.degree. or less of the circumference of the part in order to form a deposit that resists part disassembly, but does not interfere with or alter the final assembly of the parts. The present invention further contemplates a method of forming more than one retaining element either of the same or different types on a single fastener or other discrete article and fasteners with such retaining elements applied thereto.